The present invention relates generally to a method for manufacturing .beta. silicon carbide whiskers according to a desired heating schedule and, more particularly to a heating schedule where the furnace atmosphere temperature is raised to at least the whisker growth temperature before being cooled to or below the growth temperature for a period to induce nucleation of silicon carbide crystals at the catalyst sites at a desired point in time which results in desirable morphologies for the crystals produced.
Silicon carbide whiskers are valued for their needlelike single crystal structure which leads to such excellent properties as high strength, high elastic modules, heat resistance, chemical stability, and so on. The whiskers have been used as a composite reinforcing material for metals, plastics, and ceramics. The most desirable whiskers are .beta. silicon carbide single crystals which have a high length to diameter ratio.
Previous methods for growing silicon carbide whiskers have generally heated the atmosphere of a growing zone inside a furnace until the growth temperature has been reached. Temperatures could fluctuate up and down during the heating process, but no effort was made to deliberately reduce the atmosphere temperature after growth temperature was reached. In U.S. Pat. No. 4,504,453 issued to Tanaka et al., trays containing materials for growing silicon carbide whiskers were moved from one temperature zone into the next higher temperature zone of a furnace partitioned into zones with varying temperatures. The trays were then held in the temperature zones for typically an hour or two. Again, no effort was made to reduce the temperature of the growth materials after growth temperature was reached, but before the whiskers had completed growing.
In U.S. Pat. No. 3,053,635 issued to Shockley, a method for growing silicon carbide crystals from a molten metal or alloy containing silicon and carbon in solution was described. A silicon carbide seed crystal was inserted into a molten bath which was at the saturation point for silicon carbide. The seed cooled a surrounding portion of the bath causing that portion to become supersaturated and, as the seed was withdrawn from the solution, silicon carbide precipitated on the seed. However, no information was given as to how this process could be applied to a gaseous system where the silicon and carbon components were not in a molten bath.
Overall, a need still existed for a method to more quickly and precisely (in time) induce nucleation at catalyst sites. Although heating the furnace atmosphere to the growth temperature and holding thereat promoted nucleation of silicon carbide whiskers at the catalyst sites, the time needed for nucleation to occur, especially at low partial pressures of the reactants, lengthened the time the furnace had to be maintained at growth temperature, and hence increased the amount of energy needed to grow a given quantity of silicon carbide whiskers. It was necessary for the melted catalyst particles to absorb sufficient silicon and carbon after reaching the growth temperature to produce the supersaturation required for nucleation. Such supersaturation can be achieved when a catalyst composition progressively absorbs Si and C from the vapor phase, doing so isothermally at the growth temperature, until it reaches a composition where the supersaturation is great enough that nucleation will occur. Additionally, because it is difficult to regulate the isothermal approach to nucleation, unwanted whisker morphologies such as too fine or thicker bent growth may be produced in undesirable proportions. Silicon carbide whiskers with these morphologies are not suited to certain commercial uses.